How to place minimum steel in a rectangular footing? 14

[pob11646]

I have got a question about the placement of minimum temperature and shrinkage steel, say for a 35" thick rectangular footing 14 feet long and 5 feet wide. Let's say, that all I need for my footing is minimum temperature and shrinkage steel.

Hence, Ast = 0.0018bh = 0.0018 * (14*12) in * 35 in = 10.58 in2.

Say, I am using #7 bars, thus, I will need 10.58 in2/0.60 in2 = 18 # 7 bars.

Method 1: Or, say I place bars in the top and bottom layers. For my bottom layer, do I need to place 14 #7 parallel to the short side, and 4 # 7 parallel to the long side. And do the same for the top layer.

Or, Method 2: I still place bars in both the top and bottom layer. For the reinforcment parallel to the do I just need to place a total of 18 #7 in both the top and bottom layers, say 9#7 in the bottom layer, and 9#7 in the top layer.

For Method 2, the Ast required parallel to the long side will be 0.0018 * (5*12) in * 35 in = 3.78 in2, or 7 # 7 bars. Do I place a total of 7#7 bars in both the top and bottom layers, say 4#7 in the bottom layer, and 3#7 in the top layer.

Method 3. Not to confuse matters, but can I place reinforcement in the bottom layer alone, say a total of 18# 7 bars, say 14#7 parallel to the short side, and 4#7 parallel to the long side.

Please advise whether Method 1 or Method 2 is more appropriate. And is Method 3 practical?

 

[pob11646]

Is anyone willing to take a stab, that if all my combined footing requires is minimum flexural steel, then why the reinforcement cannot be "split" between the top and bottom faces? What is, or are the main reason(s) for this? Do the top and bottom bars, say parallel to the long direction, not act together to resist bending about the short direction? Looking forward to your feedback and advice.

 

[StructGen]

Structural EIT, I have a detailed discussion with Dr S. K. Ghosh ACI commitee 318 member. As per him 0.0018bh is not meant to prevent sudden faliure, it is only there for temprature and shrinkage, please see ACI 318-08 R15.10.4, it is clearly stated there.

 

[hokie66]

pob,

The answer to your latest question is definitely NO. The top and bottom bars do not work together to resist the bending. Either the top or bottom is in tension, not both, and only the bars on that face resist the tension. That is the main difference between flexural reinforcement and "T&S" reinforcement. The assumption in providing T&S reinforcement is that the whole thickness of the section is in the same stress state, so the reinforcement can be evenly distributed. As stated before, it is unfortunate that the codes use the same percentage of reinforcement for minimum flexural reinforcement as for T&S reinforcement, but the only similarity is the percentage, not the function.

 

[pob11646]

Thank you so much for your response, hokie66. It really helps me to understand the underlying principles behind the footing design.

Ok, you said, "The assumption in providing T&S reinforcement is that the whole thickness of the section is in the same stress state, so the reinforcement can be evenly distributed."

Follow up Q1:
Do you mean that if only Temperature & Shrinkage reinforcement is required, then we can, or that we are allowed to, use the assumption that the whole thickness of the section is in the same stress state, i.e. that the footing is either in tension on both the top and bottom faces, or in compression on both the top and bottom faces, and therefore, we can evenly distribute the reinforcement between the two faces, i.e. that we can split the T&S reinforcement between the top and bottom faces of the footing? Is this a "Yes" or "No"? This is basically the gist of my very first post. Please advise.

You also said, "The top and bottom bars do not work together to resist the bending. Either the top or bottom is in tension, not both, and only the bars on that face resist the tension."

Follow up Q2:
In this case, do you mean that for the minimum flexural reinforcement, the top and bottome bars do not work together because either the top or bottom face is in tension, not both, and only the bars on that tensile face resist the tension. Therefore, it is NOT permitted in this case to split the minimum flexural reinforcement between the top and bottom faces. Also, it is NOT permitted to split the minimum flexural reinforcement in both directions, meaning that minimum flexural steel has to be provided on each face, top and bottom and in each direction? Again, is this a "Yes" or "No". Please advise.

Thank you so much to all respondents who have contributed to making this topic a little clearer for all.

 

[hokie66]

Q1...if a concrete element is in direct tension, i.e. in tension over its entire cross-section due to restraint forces, then the T&S provisions apply, and the reinforcement can be distributed equally. If a concrete element is in compression across the cross-section, then the reinforcement is not for T&S, so other provisions apply. Other than continuous wall footings, T&S reinforcement does not normally apply in footings, as flexural reinforcement controls.

Q2...Yes

 

[Lion06]

StructGen-
I'm not sure why the addition of 15.10.4 was needed. I don't see any new information there. 10.5.4 still says that you need a minimum of 0.0018bh of TENSILE reinforcement. I don't believe that 15.10.4 overrides this.

 

[pob11646]

I think I just had a light bulb moment! All of us have been going on about "minimum steel", starting with my initial inquiry about the placement of "minimum T&S" steel. However, most of the time, in the subsequent posts, we do not actually specify whether we mean "minimum flexural steel, i.e. minimum main reinforcement steel", or "minimum T&S" steel.

This is going to be a long one, so please bear with me.

StructuralEIT mentioned, "TENSILE reinforcement not less than 0.0018bh shall be provided." That's "minimum flexural reinforcement".

herewegothen referred to, "minimum in tension face, i.e. all in bottom or all in top (or both to keep it simple). Shrinkage and thermal in both faces as required. All in both directions." Referring to "minimum flexural reinforcement." herewegothen's later reference to "shrinkage and thermal in both faces as required" confirms that.

Dave Atkins's "Minimum steel should not be divided into perpendicular directions." would refer to "minimum flexural steel."

hokie66's post is the one that really brought this issue to the fore, by stating, "It is unfortunate that codes continue to refer to T&S minimums when minimum flexural steel is required."

Earlier, hokie66 said, " ... if minimum steel controls, you need 7-#7 in the long direction, 18-#7 in the short direction. If this is a column which only sees gravity load and not uplift, I would only use bottom steel. I wouldn't use top steel in a footing unless required by analysis. When you do use top steel in a deep footing, the concrete usually requires revibration to prevent plastic settlement cracking." Reference to "minimum flexural reinforcement."

ACI 318-08 Section 10.5.1 states that minimum reinforcement for flexural members shall not be less than that given by As (min) = (3*SQRT(fc')/fy)*bw*dm and not less than 200*bw*d/fy. For fc' = 4000 psi and fy = 60000 psi, the minimum flexural steel works out to be 0.0033*bw*d.

ACI 318-08 Section 7.12.2.1 (b) states that the "area of S&T reinforcement shall provide at least the following ratios of reinforcement area to gross concrete area, i.e. 0.0018 in slabs where Grade 60 deformed bars or welded wire reinforcement are used. where slabs , but not less than 0.0014.

To add to this, ACI 318-08 Section 10.5.4 (per StructuralEIT's latest post) states that, "for structural slabs and footings of uniform thickness, As (min) in the direction of the span shall be the same as that required by 7.12.2.1, i.e. 0.0018*bw*d.

Section 15.10.4 states, "minimum reinforcing steel in nonprestressed mat foundations shall meet the requirements of 7.12.2 in each principal direction."

Actually, the requirements of Section 10.5.4 and 15.10.4 seem to supersede that of Section 10.5.1, as StructuralEIT pointed out.

I think it is very clear from these ACI 318-08 sections then that the minimum flexural steel is 0.0018*bw*d.

hokie66 did point out it was unfortunate that the "minimum T&S" reinforcement is also 0.0018*bw*d.

Per hokie66's latest post, most situations call for at least minimum flexural steel, 0.0018*bw*d, and this must be placed on each face and in each direction.

However, if only "minimum T&S" reinforcement is required, then we are llowed to, use the assumption that the whole thickness of the section is in the same stress state, i.e. that the footing is either in tension on both the top and bottom faces, or in compression on both the top and bottom faces, and therefore, we can evenly distribute the reinforcement between the two faces, i.e. that we can split the T&S reinforcement between the top and bottom faces of the footing?

The question now is, when do we deduce we need "minimum flexural steel", and when do we say, only "minimum T&S" reinforcement is required? This is important because it will affect the placement of the reinforcement.

Elaborating further, when can we say we need "minimum flexural steel", and therefore, we must place at least, Ast = 0.0018 * bw * d on the tensile face, on each face and in each direction.

Is there a situation where we can condifently deduce that the footing only requires "minimum T&S" reinforcement, and therefore, we can use the assumption that the whole section is in the same state of stress, and therefore split it between the top and bottom faces?

Sorry for the extremely long post, but I hope it puts everyting in perspective and bring about a discussion that would clarify this matter further. Thank you so much to all who have contributed to the the understanding of this topic.

 

[hokie66]

Yes, it is long. And some of it indicates that you understand, and some that you don't.

The amount of flexural reinforcement you require is determined by standard ultimate strength design theory, and is further limited by the minimum provisions you listed above. I thought you knew how to do that. If your footing only requires minimum flexural reinforcement, it of course requires that reinforcement on the tensile side, not the compression side. And that amount of reinforcement can also be considered as T&S reinforcement. Now, if your code (some do) required a greater quantity of T&S reinforcement due to a high degree of restraint of the element (not likely for footings), then you can count the flexural steel as part of the T&S requirement.

 

[StructGen]

Structural EIT,

Not any more, please check 05 or 08 code, they have replaced the word "Tensile reinforcement" by "Asmin" further I have discussed this with the person who wrote this clause, he was very clear that 0.0018bh is not flexural min. steel. You can run a quick calculation of 8000 to 12000 PSI conc. beam with 0.0018bh your Phi-Mn will be less then Mcr.
While Eq 10-3 gaurntees that your Phi-Mn will be greater then Mcr.
Code is very clear on this issue, either you use Eq 10-3 or you provide enough to avoid faliure. This thought is also expressed by reducing Phi for compression controled sections and Plain concrete.
If all of this above does not satisfy you let me know I will post the latter from Dr. S. K. Ghosh.


Thanks

 

[hokie66]

I would like to see Ghosh's letter. I agree with SEIT. If the amount of reinforcing to avoid failure is very low, the minimum amount is referenced to the T&S provisions. Not logical, but that is the way I read the ACI provisions. The Australian code AS3600, which did have the same connection between tensile steel and T&S, has now been changed to avoid this confusion.

 

[StructGen]

Let me raise one last counter argument before I present Dr. Ghosh latter, if 0.0018bh is the absloute minimum flexural reinforcement why it is not applicable to beams which are the primary flexural members.
This means that I can design a beam 1mx2m with less reinforcement then 0.0018bh provided that I satisfy 10.5.1, 2 & 3. While if I am designing a two-pile pile cap of 1mx2m of same load I have to satisfy 0.0018bh on tension face.
Do you think its logical?
Hokie66 do you still want me to post it?

 

[hokie66]

I don't think that using beams with such minimal reinforcement is at all practical. You have to provide 200/fy (or more for high strength concrete), but this can be reduced if you provide at least 1/3 more than required by analysis. If beam reinforcement is very light, the beam size is wrong.

A two pile cap is not a flexural element, and the minimum flexural reinforcement provisions should not be used. Truss analogy (strut and tie) is the way to go for pile caps.

In your example above, you mentioned 8000 to 12000 psi concrete. It is very unusual, if not unheard of, to use such high strength concrete in flexural members.

 

[Lion06]

I will re-read the '05 and '08 versions, but I believe that the min beam AND slab reinforcement are in the same section (10.5) which talks about minimum FLEXURAL reinforcement. I'll read it again on Monday.

 

[StructGen]

hokie66,

Not sure what type of work you do, but that is the standard material for all mid-rise (600-ft)buildings in US. 5000-8400 PSI for slabs and 12000-8000 PSI for verticals.
Further I have just given an example, a possiable sitituation, I am not debating what method is needed to design pile cap. You have to satisfy all other applicable code requirments when you use Strut-Tie. A footing is a footing you can not get away from shrinkage or min. reinf. requirments by designing it as strut and tie.

Getting back to the topic, StrEIT 10.5.4 is only applicable to footing and slabs of uniform thickness, it is not applicable to beams.

Attached please find my conversation with Dr. S. K. Ghosh, the reason I am posting this here is becuase I want to end this common misperception. This has caused me a lot of headche.

 

[hokie66]

StructGen,

I wouldn't call a 600 ft high building mid-rise, but maybe in New York it is.

I agree that 5000 psi and 6000 psi concrete strengths are typical for floors. You mentioned 8000 to 12000, which I have not used for floors. I would see no benefit in using such high strength concrete in floors, as the reinforcement percentage is not very sensitive to concrete strength. The only reason I can think of for using higher strength concrete in a floor is to allow for transfer of column loads through the floor. The Australian Code AS3600 allows the ratio of column strength to floor strength to be 2.0, provided adequate restraint is provided. Maybe the ACI code is different in that regard.

I have read the letter from S K Ghosh, and do not agree with his interpretation of Section 10.5.4 (whether he wrote it or not). He says "Section 10.5.4 essentially says that if you have provided the minimum shrinkage and temperature reinforcement required by Section 7.12, you shall be deemed to have satisfied the minimum flexural reinforcement requirements of Section 10.5". That is clearly incorrect. Section 10.5 prescribes Asmin, minimum flexural reinforcement, while Section 7.12 prescribes minimum T&S reinforcement. Why 10.5 refers back to 7.12 for the quantity is a mystery to me, as it just confuses the issue. But clearly Asmin is flexural reinforcement, and thus must be placed at the tension face.

 

[asixth]

Apologies for posting on the end of this thread.

Hokie

Where in AS3600 is there a limit of strength grade ratio between vertical and horizontal elements and does it only apply when columns are offset. AS3600 is only written for strength grades up to 65MPa so it is unlikely that something designed to AS3600 will ever have strength grade ratios greater than 2.0. Although a 600-ft building would be classified as high-rise in Australia and AS3600 would not be applicable in the design of columns where the strength grade would likely exceed 80 or 100MPa.

 

[Lion06]

StructGen-

I still disagree. Aci 318-08, 10.5.1 says, "At every section of a flexural member where tensil reinforcement is required by analysis (i.e. the plain concrete section is not adequate - that is my interpretation which I think you would agree with), except as provided in 10.5.2, 10.5.3, and 10.5.4, As provided shall not be less than that given by As,min=......."

The way I read this is that everything covered in 10.5 is minimum FLEXURAL reinforcement, 10.5.1-10.5.4 just give the quantities for different scenarios. The fact that 10.5.4 refers the quantity for minimum FLEXURAL reinforcement back to that for T&S is irrelevant. What is relevant is that it is minimum FLEXURAL reinforcement as defined by 10.5.1.

I'm not seeing the ambiguity at all. The first few times I read it I was a little wishy washy, but the more I read it (and I've had this opinion for well over a year and a half now) the more clear it becomes to me that my above interpretation is correct.

 

[Lion06]

I left out a few things I wanted to put in the last post. First, I never said that 10.5.4 applies to beams. It is clearly stated to apply to slabe and footings of uniform thickness and that is what this post is pertaining to.

Second, I disagree with the assertion that you made in your original email to Dr. Gosh that Mcr is almost always greater than the reinforce section using 0.0018bh. I just did a quick check using a 4'x4'x16" thick footing. I get Mcr=971K-in, but the nominal moment strength using 0.0018bh with d=12.5" (assume a #8 bar with 3" cover) is 1014K-in. Neither of these values reflect the phi factor, but it is clear that the values are close. It's probably less so for thinner ftg, but more so for thicker ftgs.

Third, I didn't read anything in the email from Dr. Gosh to sway my opinion. If it's not minimum flexural reinforcement, then it should be taken out of that section. I have no change in interpretation based on the change in wording from '02 to '05/'08. Just the phrase, "... in the direction of the span..." shouts flexural reinforcement.

There is no way in the world that if a plain concrete section were not adequate, that I would feel comfortable providing tensile reinforcement of 0.0009bh on each face, even if it did work on paper.

 

[miecz]

In light of the Ghosh email to Mr. Ullah, and having reread all the previous threads on this topic, I agree with StructGen, that ACI never intended .0018bt to be the minimum reinforcing on the tension face of a structural slab of uniform thickness.

 

[hokie66]

asixth,

The section I referred to is 10.8 (b). No, it applies to direct transmission through the floor system.

True, the current Standard only applies up to 65 MPa concrete, but buildings are being built in Australia with higher strength concrete. This is allowed under Section 1.3. Subject for another discussion, but I don't know how consistently the various design offices are approaching this interim situation.